As is well known in the gas turbine engine field of technology the fuel nozzles for the engine's combustor have the propensity of coking and the severity of the problem increases as the fuel and/or compressor discharge air temperature increases. Of course, for efficient combustion and engine performance, it is desirable to operate the engine at the highest possible turbine inlet temperature in order to achieve optimum engine performance. Hence to be compatible with this prerequisite, it is abundantly important to eliminate or minimize coking to assure that the fuel passageways of the fuel nozzles remains relatively clean and free from coke build-up. Military aircraft and particularly those classified as fighter aircraft are continuously increasing their speed requirements, necessitating the increased temperature requirements imposed on the engine's components.
Obviously, this has created significant burdens and challenges on those faced with the problems of combating these high temperature requirements while at the same time providing high performing and reliable engines powering these aircraft.
It is well known that one of the methods for preventing coking is by keeping the wetted wall temperature of the fuel passageways below a maximum temperature of, say 400 degrees Fahrenheit. Since the temperature of the compressor discharge air to which the fuel nozzle is subjected may get as high as 1600 degrees Fahrenheit, a known method of maintaining the wetted wall temperature at a tolerable level is by insulating the fuel nozzle. One way that has proven to be satisfactory is by insulating the fuel nozzle by incorporating an evacuated jacket that separates the high temperature surfaces of the fuel nozzle from the lower temperature surfaces that are in contact with the fuel.
This approach at these temperature levels has been evaluated and although the evacuated jacket maintains the wetted wall temperature to acceptable levels, the thermal gradients in the fuel nozzle was shown to be exceedingly high which created exceedingly high axial stresses that could only be alleviate by use of axial bellows. The prior art exemplifies different fuel nozzle designs that incorporate axial bellows. However, not all the designs utilize the bellows to prevent undue axial levels of stress. For example, U.S. Pat. No. 4,295,452 granted to M. Lembke et al on Oct. 20, 1981 utilizes an axial bellows that serves to dampen the noise created by the fuel injection valve operation. U.S. Pat. No. 3,234,731 granted to D. J. Dermody et al on Feb. 15, 1966 utilizes a pair of axial bellows to achieve a desired sealing characteristic for its dual fuel nozzle. U.S. Pat. No. 4,409,791 granted to G. E. A. Jourdain et al on Oct. 18, 1983 likewise utilizes an axial bellows for sealing. Axial bellows are known in the art for use as density and/or viscosity compensators. Use of bellows to compensate for expansion and contraction to reduce stress on the structural components are exemplified in U.S. Pat. No. 4,258,544 granted to D. E. Gebhart et al on Mar. 31, 1981 and in U.S. Pat. No. 4,384,846 granted to R. Walderhofer on May 24, 1983.
However, none of these patents suggest the use of the combination of axial and radial bellows or the use of radial bellows by itself to solve the thermal stresses occasioned in a high temperature environment to which the fuel nozzle disclosed in the present patent application is subjected. This invention contemplates the use of radial bellows to solve the thermal stress problems.
In investigation of the present invention it was found through structural and thermal analysis that the thermal gradient in the fuel nozzle tip exceeded 1200 degrees fahrenheit in a quarter of inch of radius. Such would be the case for any design that successfully insulated the fuel passages. However, the large thermal gradient in the tip resulted in unacceptable stress levels in the nozzle, particularly at the weld joints. This presented a serious problem since it included a weld joint that was critical to the design of the fuel nozzle.